1. Field of Invention
This invention pertains to hulls for ships, yachts, submersibles, seaplane hulls, and other water-borne vessels. More particularly, it pertains to new hydrodynamic structures, the Transonic Hydrofield (TH), which intrinsically shapes a new type of displacement form, the Transonic Hull (TH). Together, THand TH are characterized in substantially eliminating wave-making drag of displacement hulls, without incurring the drag penalties of planing hulls.
2. Description of Related Art
The potential of this writer's hull invention can be better appreciated by first reviewing the advantage and limitations of conventional hulls.
2.1 Displacement Hulls. Displacement hulls sustain boat weight by buoyant lift. As designed in the past and present, they have an upper speed limit called "hull spaced," above which hydrodynamic resistance (drag) grows at a high exponential rate. "Hull speed" varies inversely with on the square root of waterline length. Displacement hulls arc very efficient well below hull speeds, having a weight-to-drag efficiency ratio of the order of 80. At very low speeds (creep), the efficiency ratio increases to much higher values, because drag approaches zero but weight remains constant. However, near or above hull speed, their weight-to-drag ratio becomes physically and economically unacceptable. Therefore, greater speed of displacement hulls is attainable principally by increasing hull length. However, the advantage of length is not large. For example, the hull speed of a 50 foot hull is 9.5 knots, but for 300 foot hull speed, it is only 23 knots.
The hull speed limit in intrinsic of displacement hulls, because of their wave generation properties as they translate in the water, i.e., "wave making," which becomes critical at hull speed, as will be reviewed later on. This is a very serious problem in the economics of maritime transportation. For that reason, considerable research has been done in various ways to overcome it, unfortunately with only minor improvements. For example, a bulbous bow may slightly decrease drag at certain speed. Also, long slender hulls are less sensitive than beamy hulls, but carry less cargo, and have other problems, as will be reviewed later on.
Accordingly, there remains an urgent need for improving the speed range and high speed efficiency of displacement hulls. A practical solution of this problem is needed, especially if it is able to eliminate wave-making drag without recourse to hydrodynamic planing.
2.2 Planing Hull. There is a widely held view that the planing hulls, in which weight is supported principally by a hydrodynamic lift force from momentum change (as distinct from buoyant lift), can overcome the speed limits of displacement hulls, and furthermore that they are efficient at high speed. Actually, while planing permits high boat speed, it does so only for boats with an approximately flat underbody having relatively light weight and equipped with large propulsive thrust. But the fact remains that planing is a grossly inefficient hydrodynamic regime, since the best ratio of boat weight to resistance is only about 8. This is less than half that of a modern jet transport flying about 10 times faster, and only 1/5th that of a displacement hull of "reasonable" length near, but below, hull speed. The limitations of planing are inherent in their generation of lift by angle of attack, described mathematically with equations which are analogous to those of supersonic flight, as will be reviewed later on for the limiting case of inviscid planing.
2.3 Semi-Planing Hulls. Unlike displacement hulls which have upwardly curved sterns and curvatures at the bow, causing their CG to sink with forward speed (increasing their apparent weight), and unlike planing hulls having flat undersurfaces and a CG which tends to rise with forward speed, the semi-planing hull usually has a Vee bottom and, for practical reasons, is heavier than a pure planing hull. Although the semi-planing hulls can generate the appearance of a "flat" wake at high speeds, their lift is generated by a combination of buoyancy and dynamic forces, which is very inefficient. The borders of their "flat" wakes, as seen from an aerial view, join together at some distance behind the stern, generating a trailing "hollow" on the water's surface, which can be interpreted, from the viewpoint of a fish trained in hydrodynamics, as an virtual displacement hull of larger length than the waterplane of the semi-planing hull. The semi-planing hull is an inefficient hybrid at slow speeds, it has excessive drag compared to a displacement hull. It requires very large power to reach semi-planing speed, at which regime it is less efficient than a pure planing hull. On the other hand, a semi-planing hull provides smoother ride for a greater payload in a rough sea, and is probably more seaworthy than a planing hull. However, it has a rougher ride than a displacement hull, with less favorable sea keeping characteristics, and is commercially not viable for most maritime applications.
2.4 Semi-Displacement Hulls. As length-to-beam ratio is increased in slender hulls, wave-making drag decreases. According to Saunders, slender displacement power boats were common in the 1910s. Later on, the German Schnell boote (fast boat), having a round-bottom hull, was successfully developed as an S-boat for WWII, performing well at high speeds in the rough North seas. However, as the length-beam slenderness ratio of semi-displacement boats is further increased, the lateral stability and payload capacity is further decreased. In the extreme, an 8-man rowing shell relies on oars for lateral stability. With a length-to-beam ratio of 30, its wave-making resistance is only 5% of the total at 10 knots, but its weight-to-drag ratio is only 20, approximately. An appropriate comparison in aircraft is the modern sailplane with a wing span-to-chord ratio of 25. It can operate at weight-to-drag ratio of 40, at 6times the speed.
In the limit as beam of slender hull approaches zero, wave-drag tends towards zero, but viscous drag subsists and payload capacity vanishes. Accordingly, recent development of high speed semi-displacement boats have proposed a mixed lift mode, using complex additions to the hull to generate hydrodynamic lift at higher speeds, in order to decrease buoyant lift component, and to compensate other shortcomings of the slender hull at high speeds, for example, lateral instability and a tendency for nose high attitude. As is the case for semi-planing hulls, their ratio of weight-to-drag is not very satisfactory, and in consequence, payload is not large. Although they appear to have performance advantages over semi-planing near hull speeds, and are less sensitive in pitch and their complex shapes appear to have an inherent size limit. It may be added that the proper name for this kind of vessel should be displacement-dynamic hull or quasi-displacement hull, rather than semi-displacement.
2.5 Multi-Hulls. The wave-making and other drag problems of the various hull types reviewed above are so serious that considerable recent efforts have ben applied for the development of new multihulls. Although this field is outside the scope of this review, a few remarks are in order. A pair of very narrow slender displacement hulls of a Catamaran, widely spaced laterally for stability, have been successfully developed and are being used at high speed for various applications, especially in Asia. SWATHS are also multihulls which rely on totally submerged primary displacement for performance and smooth riding. These developments and other high speed hull developments (see, for example, Jane's high speed marine craft) have so far been restricted to special applications, highlighting the need for ship manufacturers to increase the speed and improve the riding qualities of displacement monohulls.